1. Field of the Invention
This invention relates to optical amplifiers and, in particular, but not exclusively, to a rare earth doped fibre amplifier with improved operating characteristics.
2. Related Art
Rare earth doped optical fibre amplifiers are ideal as repeaters, pre-amplifiers or the like in optical communication systems. The desirable features of such amplifiers include low noise operation, polarisation insensitivity and low insertion loss. One particular advantage derived from using rare earth dopants is that they typically exhibit sharp spectral features. In contrast, transition metal ion-doped media exhibit very broad absorption and fluorescence features. Of the rare earth dopants, erbium, when used as a dopant in silica fibre, has lasing properties in the desirable 1550 nm window for optical communications, and similarly, praseodymium operates in the desirable 1300 nm window. For convenience only, erbium and praseodymium doped silica fibres will be referred to in the following description, although it is emphasised that the description applies equally as well to other rare earth dopant species and host configurations.
The relatively long fluorescence lifetime of the upper state of the amplifying transition in erbium, compared with for example semiconductor laser transitions, has many important implications. One implication is that electrical noise on the bias supply to a semiconductor pump laser used to pump an erbium doped fibre amplifier introduces a degree of modulation on the gain of the amplifier at low frequencies of pump noise components below 100 KHz. Therefore, electrical bias supplies for pump lasers need to be noise free at these low frequencies at least. Another implication is that the propagation of a signal through an erbium doped fibre amplifier can cause modification of the population inversion and therefore a modification of the amplifier gain for its own and other wavelengths. This effect is particularly marked for pulses that are sufficiently intense to saturate the gain. Severe pulse shaping can occur as a result of this process.
In multi-wavelength multiplexed transmission, for example wavelength division multiplexed (WDM) systems, modulation of the amplifier gain by one of the multiplexed signals can result in low frequency crosstalk effects on the other signals. These effects are particularly significant, where the transient effects associated with turning channels off and on can seriously disrupt other wavelengths. This effect can be eliminated if the amplifier's gain, and hence gain spectrum, is controlled independently of input signal level.
Known systems for implementing independent amplifier gain control use automatic gain control (AGC) in the form of opto-electronic or all optical feedback loops, where the all-optical option is more desirable in terms of reduced complexity and cost. Also, opto-electronic feedback loops suffer with limited speed of response and potential degradation of the amplifier noise response.
One method of making an amplifier gain independent of input signal using an all-optical feedback loop has been proposed in European Patent Application 92300519.3. The method describes a semiconductor pumped erbium doped fibre amplifier (EDFA) which has coupled to it an optical feedback loop which couples the output of the amplifier to the input of the amplifier. A narrow bandwidth filter coupled to the feedback loop allows selected control wavelengths of the amplified spontaneous emission to pass from the output of the fibre amplifier to the input of the fibre amplifier. The feedback signal has a control wavelength which is different from that of the pump and the wavelengths of the signals to be amplified. The feedback control signal in effect locks the amplifier in a ring laser configuration. Thus, lasing conditions are controlled by the wavelength of the feedback control signal and the attenuation in the feedback loop, and not the input power of the pump or signal wavelengths. The ring laser configuration necessitates the use of a wavelength selective coupler (WSC) and three 3 dB couplers or alternatively three WSCs, which, in terms of manufacture, is a complex arrangement.
An alternative arrangement is proposed in "Gain control in erbium-doped fibre amplifiers by lasing at 1480 nm with photoinduced Bragg gratings written on fibre ends", Delevaque et al., Electronic Letters, June 10th, Vol 29, no. 12. The arrangement involves writing Bragg reflectors at a control wavelength at both fibre ends of an EDFA, there being no intrinsic loss mechanism for the signal except for the reflectors and for coupling losses due to fusion splicing.